The present invention relates to an electro-acoustic transducer which can be used both as an emitter and as a pick-up for acoustic waves in an ambient medium.
Since the physical phenomena (electromagnetism, electrostatics and piezo-electricity) which are utilized for producing electro-acoustic transducers are reversible, it is in principle possible to use the same piece of equipment for emitting sound waves in response to an electrical signal and conversely for converting acoustic vibrations into electrical current or voltage variations. In practice, however, transducers intended for use as microphones and as loudspeakers which operate on the basis of a given principle are not constructed in the same manner. The dimensions, shape and arrangement of the components forming the device differ in the two cases. For example, in order to be sufficiently responsive to sound waves, the diaphragm of a microphone must be much less rigid than that of a loudspeaker.
Therefore, providing a piece of equipment with the facility to emit and receive sounds requires the joining of both a loudspeaker and a microphone which are separate from each other, each transducer having characteristics selected to adapt it optimally to its respective function, taking into account the inherent properties of the transducer type.
This gives rise to a problem when dealing with portable devices which are to be of minimum size. This is the case in particular with watches which are designed on the one hand to indicate the time or provide other items of information, either in the form of simple single-tone sounds or in a "spoken" form by means of words or groups of words which are stored in a memory, and on the other hand have functions, such as altering the time display, which can be controlled directly by use of the voice. The fact that such watches have to include two independent transducers, in addition to the usual components required to derive and display the time, militates against the efforts which are being made at the present time to reduce the size of such watches.